Kaoru AoiGet started with changing your game.
The default template starts you off with an animals template that you'll probably want to switch to your own theme.
Basic changes to your game name and description can be found by clicking on the Admin menu on the top.
After renaming your game, you may head over to the marketing page on the second level top menu. There you can explain your game in more depth.
The gameplay admin page contains even more customization options. You'll want to change the backgrounds of your board, card backs, deck sizes, etc when you're ready. Changing the display names of card types is recommended but changing the IDs is not. IDs are referenced and used by game rules so changes to IDs will break whatever template you started with.
At the bottom of the gameplay admin page are the settings for keywords and properties that are defined on the card types. Changing the display names and help text here lets you easily reskin any template without touching any rules.
The Admin submenu has even more pages for customization, including market, audio, achievements, gamemodes. Enjoy.